1. Field of the Invention
The present invention relates to a node, which supports enhanced link sets, having the ability to transfer longer messages than according to current MTP level 2, wherein the node includes, in addition to a standard first point code, a second point code which enables the full use of the longer message length.
2. Description of the Prior Art
FIG. 1 shows the various protocol stacks for SS7 (Signalling system no: 7) up to the MTP (message transfer part) level. Five stacks are currently defined. The first stack is the well known stack for operation on 56/64 kbit/s links.
Due to an increased bandwidth delay product, the MTP level 2 (Q.703) is not ideally suited for speeds significantly above 64 kbit/s. The elements which are problematic are window size, retransmission strategy, and the error rate monitor.
Three different protocol stacks have been defined for use on T1/E1 links (1.5/2 MBit/s) which address some or all of these problematic elements.
The latest edition of Q.703 contains, as a national option, a modification to the level 2 protocol which introduces 12 bit sequence numbers and a different error rate monitor (second column). Otherwise, the procedures are not changed.
Recommendation Q.2119 defines frame-relay framing for SSCOP (Service specific connection oriented protocol, Q.2110) to be used on a raw E1/T1 link (third stack). Thus, starting at SSCOP, the complete broadband protocol stack can be used on high speed signalling links.
In addition, Bellcore defines the complete ATM signalling protocol stack starting at the ATM layer for use on T1 signalling links, with certain restrictions in the ATM layer, like not allowing multiple VCs (virtual channels) on a T1 link (column 4).
Lastly, the full ATM signalling protocol stack (column 5) also could be used in narrowband networks.
Besides the potentially vastly different link speeds (which, however, pose no new interworking problems), the main difference between MTP level 2 based and SSCOP based signalling lies in the different maximum MSU length supported.
Of course, there is no need to actually make use of the longer MSU length supported by the ATM links in an enhanced narrowband signalling network. Indeed, the existing narrowband SS7 user parts would not even make use of the longer MSU length. We note, however, that the users of the SCCP can generate messages in excess of 255 octets (the maximum data size supportable in single messages of the pre-96/97 SCCP). Such messages will be segmented before being delivered to the MTP. If such traffic would go via ATM links, avoiding the segmentation would benefit performance significantly. Therefore, the situation exists that use of the larger MSU sizes—where needed and when possible—would be an additional welcome benefit of using the enhanced linksets.
Each node in an MTP network is identified by one signalling point code. An MTP network is identified by the so-called network indicator in an MTP message. Routing in the MTP is based on the so-called destination (signalling) point code (DPC) which identifies the destination of a message signalling unit (MSU) in an MTP network. In addition, the signalling link selection field (SLS) can be used do select between available routes of equal priority (combined linksets) and to select a specific link within a linkset (a collection of links directly connecting two signalling points). No other information (like origination, MTP user, or MSU length) is generally evaluated for routing in the MTP.
The SCCP augments the MTP routing by providing additional functions to route on a so-called global title (GT), which can e.g. be a subscriber number of an 800-number. An SCCP routing on GT performs a process called global title translation (GTT) which derives the DPC of the final destination or the DPC of the next node (intermediate translator node) where the GT is further analyzed, eventually leading to the DPC of the final destination.
In addition to the GT the SCCP uses a so-called subsystem number (SSN) to identify the addressed SCCP user in the final destination.
This process also allows an SCCP message to cross MTP network boundaries.
In addition, the outcome of a GTT can depend on the availability status of the (next) destination. If the so-called primary destination, which would normally be the result of a GTT, or the addressed SSN is not available or reachable, an alternative destination can be the result of the GTT. This allows the SCCP to route messages to backup destinations (or backup intermediate translator nodes). Loadsharing between destinations is, in principle, also a possibility. Between two SCCP nodes the messages are routed by the MTP using the DPC provided by the SCCP.
State of the Art
The interworking problem arising if use of longer messages in networks containing also linksets supporting only short messages is to be made has not been addressed in any detail. Bellcore simply specifies that long messages destined for an MTP level 2 based link are to be discarded and that other routing should be administrated accordingly.
A similar solution is proposed for the MTP based narrowband-broadband interworking in Q.2210. For the SCCP, the possibility is defined to convert long LUDT(S) messages into segmented short XUDT(S) messages.
All these solutions, however, require appropriate planning of the routes supporting the longer messages and/or will not make optimal use of the capabilities available. An MTP level 3 protocol based approach to solve such problem is described in Q.701. This solution, however, is incomplete.
This invention proposes to use the addressing mechanisms provided in MTP and SCCP to solve, or rather prevent, the above-described interworking problem.